Bad Girl Bill by Diane Saxon

From the moment British Special Forces Operative Michael Marsden meets Deputy Sherriff Bill Swann, sparks fly and lust flows.
When a man meets a woman who rides a Harley Davidson and wears a gun strapped to her hip, he’s bound to be in for a few surprises. Because long-legged tomboy, Bill, is definitely not what she seems.
Having made it clear he’s looking for a no-strings-attached fling, Michael realizes he has to use all of his tactical knowledge to negotiate his way around Bill’s five older brothers to get her alone. Then he needs to get past her defenses. And his own heart.